This invention relates to removing bromine reactive hydrocarbon contaminants in aromatic streams by contacting the stream with an acid active catalyst. The aromatic streams have a negligible diene level before contacting and decreased levels of mono-olefins and dienes after contacting. Dienes may be removed in a pre-treatment step according to the invention.
In petroleum processing, aromatic streams are derived from processes such as naphtha reforming and thermal cracking (pyrolysis). These aromatic streams also contain undesirable hydrocarbon contaminants including mono-olefins, dienes, styrenes and heavy aromatic compounds such as anthracenes.
The aromatic streams are used as feedstocks in various subsequent petrochemical processes. In certain of these processes, such as para-xylene production, e.g., from an aromatic stream containing benzene, toluene and xylene (BTX) or toluene disproportionation, hydrocarbon contaminants cause undesirable side reactions. Therefore the hydrocarbon contaminants must be removed before subsequent processing of the aromatic streams.
Moreover, improved processes for aromatics production such as that described in Handbook of Petroleum Processing, McGraw-Hill, New York 1997, pp. 4.3-4.26, provide increased aromatics yield but also with an increase in bromine-reactive hydrocarbon contaminants. The shift from high-pressure semiregenerative reformers to low-pressure moving bed reformers results in a substantial increase in bromine reactive contaminants in the reformate derived streams. This in turn results in a greater need for more efficient and less expensive methods for removal of hydrocarbon contaminants from the aromatic streams.
Undesirable hydrocarbon contaminants containing olefinic bonds are quantified by the Bromine Index (BI). Undesirable olefins, including both dienes and mono-olefins, have typically been concurrently removed from aromatic streams such as BTX by contacting the aromatic stream with acid-treated clay. Other materials, e.g., zeolites, have also been used for this purpose. Clay is an amorphous naturally-occurring material, while zeolites used for this purpose generally are synthesized and are therefore more expensive. Both clay and zeolites have very limited lifetimes in aromatics treatment services. The length of service correlates with the level of bromine reactive impurities in the feedstream. BI-reactive contaminants rapidly age both clay and zeolites. Indeed, although clay is the less expensive of the two alternatives, large aromatic plants can spend more than a million dollars a year on clay. Furthermore, since zeolites are considerably more expensive than clay, their use in removing hydrocarbon contaminants can only be justified by dramatically improved stability in aromatics treatment so that their cycle length is practical.
An object of the invention is to provide a method for removing bromine-reactive hydrocarbon contaminants from aromatic streams with longer practical cycle lengths.
Another object of the invention is to remove bromine-reactive hydrocarbon contaminants from aromatic streams using crystalline molecular sieve catalysts under conditions fostering catalyst stability sufficient to provide economic incentive to replace clay for this purpose.
Yet another object of the invention is to provide a method of pretreating aromatic streams to remove dienes before removing mono-olefins.
A method for removing bromine-reactive hydrocarbon contaminants from an aromatic hydrocarbon stream comprises providing an aromatic feedstream which has a negligible diene level, and contacting the feedstream with an acid active catalyst composition under conditions sufficient to remove mono-olefinic bromine-reactive hydrocarbon contaminants.
The acid active catalyst is preferably a crystalline molecular sieve material having ten or more membered oxygen rings, more preferably a layered material.
The aromatic hydrocarbon stream to be contacted with the acid active catalyst is an essentially diene-free aromatic hydrocarbon feedstream. This feedstream may emerge diene-free from another petroleum processing procedure, or a diene-containing stream can be pre-treated to selectively remove dienes. The stream can be pre-treated by contacting with clay or a hydrotreating catalyst under conditions sufficient to substantially remove dienes but not mono-olefins.